The present invention relates generally to paper making machines and, more particularly, to press sections and press section-frame construction combinations in paper making machines.
In press sections of the type with which the invention is concerned, a paper web formed on the wire part of the paper machine is passed through a first double-felted press nip wherein dewatering of the paper web takes place through both faces of the web. The press section also includes a smooth-surfaced center roll which forms at least one single-felted press nip through which a first fabric which passes through the first double-felted nip also passes. The web separates from the first fabric at the first single-felted press nip and is carried on the smooth surface of the center roll, preferably into a second single-felted nip provided with its own press fabric. The web is then detached from the smooth surface of the center roll, preferably as an open draw, for further processing.
The frame construction to which the invention is directed includes in the direction of web run, a first front frame, a second rear frame, and, possibly, an intermediate frame, on which the press rolls are mounted.
The starting point of the invention is a further development of the Sym-Press II press section available from applicants' assignee, Valmet Paper Machinery Inc. of Finland. The construction of the Sym-Press II press section is disclosed in Finnish Announcement Publication 50,651 corresponding to U.S. Pat. No. 4,209,361.
With regard to the prior art related to the invention, reference is also made to U.S. Pat. Nos. 4,257,844 and 4,192,711, Finnish patent 72,159, and the articles in the publications Das Papier, Heft 1, pages 33 to 34, 1981, and Norsk Skogindustri, No. 3, 1974, page 80.
A modification of the Sym-Press II press section is described in the latter articles in which the suction roll does not form a nip with the smooth-surfaced center roll and wherein the first double-felted press nip in which web dewatering takes place in two directions is provided in connection with or ahead of the suction roll. Instead of the Sym-Press suction roll, a hollow-faced press roll is provided in nip-defining relationship with the smooth-surfaced center roll to form a second press nip. A third press nip is formed substantially at the opposite side of the smooth-surfaced center roll with respect to the second nip.
In the press sections disclosed in the above-mentioned publications, it has been necessary to use a suction roll either as a press roll or in order to alter the direction of the web-supporting pick-up felt upwardly towards the second nip. However, the use of a suction roll or any other corresponding suction device results in considerable drawbacks. For example, the perforated surface of a suction roll tends to leave markings in the web which are detrimental to the appearance of the paper and which may affect its surface properties. Press suction rolls are expensive, require their own drive motor and regulation system and are noisey in operation.
Furthermore, although it is desireable to provide for crown adjustment of press-suction rolls, this is generally not possible since the mantle of the suction roll is perforated and/or since the interior space within the roll is occupied by the suction equipment to such an extent that the crown adjustment devices cannot be fitted inside the roll. One of the objects of the present invention is to eliminate this drawback.
With regard to conventional frame constructions for press sections, difficulties arise in the replacement of press fabrics as well as of press rolls. These problems have become more severe as the widths of paper machines have increased due to the fact that the press rolls have become longer and heavier. The increased use of press fabrics made of pastic material which are substantially rigid have also increased the difficulty of replacement of the press fabrics since such fabrics cannot be easily folded into small bundles.
In an attempt to overcome the difficulties of replacing the press rolls and press fabics, a frame construction has been designed by Beloit Corporation, designated by it as the Flip-Top frame construction, in which a top frame is situated above the press rolls. The top frame is provided with a pivot shaft extending parallel to the transverse direction of the paper machine so that either the top part of the front frame or the top part of the rear frame can be pivoted around the pivot shaft to open a portion of the frame. However, both of the top parts of the frame cannot be opened at the same time. Opening the top part of the front frame facilitates the replacement of the pick-up fabric of the press section as well as the fabric that usually passes through the first and second nips. The top part of the rear frame is locked during such replacement to support the open top part of the front frame. In a corresponding manner, when the top part of the rear frame is pivoted to its open position, such as to facilitate replacement of the press fabric of the third press nip, the top part of the front frame must remain in its closed position to provide support for the opened rear frame. The lower fabric of the first nip, which generally runs through the basement space of the paper machine hall, is replaced by conventional arrangements. It is seen from the foregoing that the Flip-Top frame construction has a "drawbridge" type construction having a bridge or span which is opened or pivoted around an articulated joint situated at the middle of the span so that only one half of the span can be opened at a time.
Frame constructions for press sections are disclosed in Finnish patent applications 844693 and 854959, assigned to applicants' assignee, in which the front frame and the rear frame are connected to each other and/or to an intermediate frame on which the center roll is mounted, by means of various openable and closeable intermediate frames situated above the front and rear frames.
When press roll arrangements having several nips are operated, space problems are usually encountered due to necessity of accommodating several different press rolls and their associated equipment in a relatively limited space. For this reason, it has been necessary to locate the frame parts that connect the front and rear frames to each other or to an intermediate frame, with their opening and closing means, in highly congested areas. This results in problems in both the construction and operation of the press section. An object of the invention is also to provide solutions for these problems.